Derived from the ancient BSD Unix, *BSD has a set of queue related macros which are described in queue(3), and implemented in src/sys/sys/queue.h. These are primarily used in the kernel, but also useful for userland applications.
Some use of queue(3) in the current FreeBSD kernel could be improved so that the readability would be improved. E.g. determining whether the queue head is NULL, could be replaced with FOO_EMPTY() macro. I am looking for someone to work together with me in the way of peer-review so we can make sure the code is correct before committing against the central FreeBSD repository.